Oil and gas lubricating devices are widespread and are used, for example, for supplying lubricant to high-speed spindles in modern machine tools. An unnoticed malfunction of the lubricant supply can lead to a malfunction of the device to be lubricated, e.g., a machine tool, and therefore to high consequential costs and problems, such as loss of production, for example. This clearly shows the need for a capability of automatically monitoring the supply of lubricant by an oil and gas lubricating device, because a subjective, visual monitoring, for example, by an operator, is too uncertain. The customary oil and gas lubricating devices are minimal quantity lubrications, in which the oil, instead of flowing in streams, flows only on the order of individual drops. These drops are pulled apart by an airflow and, in the form of a fine oil film, also called striae, conveyed along a wall of a supply line in the direction of a lubrication point, e.g., a spindle bearing. Such a limited quantity of oil hinders the monitoring of the lubricant supply by the oil and gas lubricating device that can be carried out with a justifiable effort.
DE 44 39 380 A1 discloses a lubricating device for minimal quality lubrication in which a sensor monitors the lubricant flow and records a temporal change in the lubricant flow. For example, the sensor can be a light barrier, whose detection beam penetrates the lubricant flow and the transparently constructed supply line thereof diametrically and whose detection beam is characteristically changed by the lubricant flow. A photoreceiver generates an electric striae signal that is representative of the lubricant flow or the striae and from which the presence and quality of the lubricant flow is derivable.
A further example for a method for monitoring an oil and gas lubricating device from the state of the art is described in WO 01/36861. In the method described in this publication, the temporal change in the lubricant flow is also registered by a striae sensor, and the striae sensor outputs an electrical striae signal that corresponds to the fluctuations in the lubricant flow.
Detrimental in the monitoring of an oil and gas lubricating device using the methods described in DE 44 39 380 A1 and WO 01/36861 is the faultiness of the striae signal as a result of strong, non-representative fluctuations and peaks. As a result, the significance of the striae signal is reduced, because a temporarily low striae signal is not unusual and also does not necessarily have to be connected with a fault in the oil and gas lubricating device. One risk that arises is that a malfunction in the oil and gas lubricating device may not be detected quickly enough, because the low striae signal is not sufficiently observed. On the other hand, it is also possible that the lubricated system is incorrectly switched off because of the temporarily indicated low striae signal, although the oil and gas lubricating device is working correctly. In both cases, unnecessary costs are created. The usability of the striae signal is accordingly considerably reduced as a result of its faultiness.
Furthermore, automatic evaluation of the striae signal is difficult because specifying a predetermined limit is made more difficult because of the faults in the striae signal.